Mercury and Venus

Reaching the innermost planet was a tough challenge for NASA, but techniques used there paved the way for future missions elsewhere. Meanwhile, the Soviet Union finally reached the surface of Venus.

Mercury was always going to be a difficult world to reach, simply because of the immutable laws of planetary motion. With an average distance from the Sun of just 58 million km (36 million miles), Mercury has a year that lasts just 88 Earth days. The tiny planet, little larger than our Moon, moves through space at around 48km (30 miles) per second. Earth, in contrast, moves along its orbit at 30km (19 miles) per second.

For a spacecraft launched from Earth to catch up with Mercury in its orbit, it would have to pick up a huge amount of speed, and when NASA turned its attention towards Mercury in 1968, there was simply no way that even a lightweight probe could achieve such a speed with existing rockets. Fortunately there was an alternative - why not put the Mariner 10 spacecraft into an elliptical orbit around the Sun, designed so that it would orbit once for every two Mercury years, coming

SOLAR SAILOR

When Mariner 10 ran out of fuel, ground controllers used the pressure of solar wind particles on its solar panels to steer the probe to its final flyby.

MAPPING MERCURY

This image mosaic was compiled during Mariner 10's finol flyby. Because the probe met the planet every two Mercury years, the same areas were illuminated on each flyby.

TECHNOLOGY

THE JOURNEY OF MARINER 10

close to the planet at its perihelion point not once but perhaps several times? Such an orbit would require far less energy, making the mission possible, but putting Mariner 10 on its elliptical course would mean using an untried theoretical technique - a gravity assist or "slingshot" from Venus (see panel, below). So Mariner 10's mission would not only give scientists a first close-up look at Mercury, it would also act as a rehearsal for later missions using the slingshot technique, such as the TOPS probes (see p.264).

The spacecraft made its first rendezvous with Mercury in March 1974 and sent back images of a cratered, baking world devoid of atmosphere,

Mariner's flight path

Venus flyby: February 1974

Mercury flybys: March 1974, September 1974 March 1975

launch from Earth, November 1973

In order to reach Mercury, Mariner 10 had to enter a precise elliptical orbit with a period of 176 days (two Mercury years), intersecting not just with Mercury's orbit but with the planet itself. It did this by "borrowing" energy from Venus to speed up and change course. As it approached Venus in February 1974, the probe was pulled in by the planet's gravity, accelerating so that it left the encounter on a new elliptical path that carried it closer to the Sun. Mariner 10 made three encounters with Mercury before it lost the ability to manoeuvre.

IMAGES FROM THE SURFACE

The thick Venusian atmosphere did such a good job of swamping radio signals from the planet's surface that the later Soviet probes used a different strategy to earlier missions. On Veneras 9 through 14, the landers were carried to the planet by a mothership that remained in orbit while they descended to the surface. The mother ship acted as a signal relay, picking up the data sent from the lander below and re-transmitting it to Earth through clear space. The monochrome and later colour images revealed a Venusian surface scattered with apparently volcanic rocks on a darker layer of "soil" - in fact it now seems that almost all of the surface material originated in volcanic eruptions.

18 October 1967

Venera 4 makes the first Soviet attempt to soft-land on Venus.

15 December 1970

Venera 7 lands successfully on Venus.

3 November 1973

Mariner 10 leaves Earth en route for Mercury.

5 February 1974

Mariner 10 flies past Venus, making the first gravity-assist manoeuvre.

29 March 1974

Mariner 10 makes the first flyby of Mercury.

16 March 1975

Stabilized by "sailing" the solar wind, Mariner 10 makes its third and final Mercury rendezvous.

IMAGES FROM THE SURFACE

The thick Venusian atmosphere did such a good job of swamping radio signals from the planet's surface that the later Soviet probes used a different strategy to earlier missions. On Veneras 9 through 14, the landers were carried to the planet by a mothership that remained in orbit while they descended to the surface. The mother ship acted as a signal relay, picking up the data sent from the lander below and re-transmitting it to Earth through clear space. The monochrome and later colour images revealed a Venusian surface scattered with apparently volcanic rocks on a darker layer of "soil" - in fact it now seems that almost all of the surface material originated in volcanic eruptions.

Veneras 9 and 10 had a two-part design, with the lander released by a carrier spacecraft that went into Venusian orbit. These landers were able to send back the first black-and-white images of the planet's surface (see panel, above). Veneras 11 and 12 used a similar technique, delivering information about the atmosphere during their descents and revealing that Venus has lightning. Veneras 13 and 14, which touched down in 1982, sent back higher quality, colour pictures, while the last of the series, /j Veneras 15 and 16, were orbiters that used radar to map the planet through y2B6| the clouds. This technique was first fUfSii8 used by NASA's Pioneer IQfl

Venus 1 which orbited the oljUB

planet for 14 years from 1978. Initially accompanying it was Pioneer Venus 2, which launched k^^jM a series of four probes to briefly monitor the Venusian /S atmosphere.

18 October 1967

Venera 4 makes the first Soviet attempt to soft-land on Venus.

15 December 1970

Venera 7 lands successfully on Venus.

3 November 1973

Mariner 10 leaves Earth en route for Mercury.

5 February 1974

Mariner 10 flies past Venus, making the first gravity-assist manoeuvre.

29 March 1974

Mariner 10 makes the first flyby of Mercury.

16 March 1975

Stabilized by "sailing" the solar wind, Mariner 10 makes its third and final Mercury rendezvous.

22 October 1975

Venera 9 returns the first pictures from the surface of Venus.

16 November 1978

Pioneer Venus 2 releases the first of its probes into the planet's atmosphere.

4 December 1978

Pioneer Venus 1 arrives in orbit to make the first radar maps of Venus.

10 October 1983

Venera 15 arrives at Venus with a powerful radar mapping system.

RADAR MAPPER

Veneras 15 and 16 carried an early version of the synthetic aperture radar used by Magellan. Once in orbit, the solar panels unfolded and the cone at the top opened to form a radar dish.

VENUSIAN ATMOSPHERE

Mariner 10's ultraviolet camera revealed weather patterns in the clouds of Venus for the first time - and in 1979 the Pioneer Venus 1 orbiter took the picture above. The orbiter was combined with a multiprobe mission (right) which deployed probes to study the atmosphere in detail.

but with many features suggesting that it has had an unusual history. Perhaps the most spectacular discovery was the enormous Caloris Basin, an impact scar some 1,300km (800 miles) across. In September 1974 and March 1975, Mariner 10 made two further flybys of Mercury, successfully photographing some 45 per cent of its total surface. However, the nature of the probe's orbit made it impossible to map the rest of the planet, and Mercury was temporarily abandoned (but see p.306).

Landings on Venus

The Soviet Union's attempts to explore Venus did not begin well - Venera 4, which entered the atmosphere in October 1967, and Veneras 5 and 6 (May 1969) were all lost during their descents. But when the probes were shielded with increasingly heavy armour against the hostile conditions, their luck started to change. Venera 7 was the first probe to make it all the way to the surface -communications failed as it parachuted into the acid skies in December 1970, but when the radio noise that followed was analyzed later, a faint signal was discovered. It showed that the rising atmospheric pressure had levelled off at 90 times that on Earth, while the temperature was a constant 475°C (887°F). The lack of any change revealed that this was the first signal from the surface of another planet.

With confidence in the programme reinforced, 1972's Venera 8 carried an improved radio system. This time, full contact continued all the way to the surface, although the probe stopped working 50 minutes after landing in the harsh environment.

1971

9 May 1971

Mariner 8 is launched from Cape Canaveral, but crashes when its launch vehicle fails.

30 May 1971

Mariner 9 is launched successfully toward Mars.

14 November 1971

Mariner 9 enters Martian orbit during a dust storm.

December 1971

The first usable pictures are returned from Mariner 9.

27 October 1972

Mariner 9 is switched off after exhausting the fuel in its attitude control motors.

20 August 1975

Viking 1 launches from Cape Canaveral.

9 September 1975

Viking 2 launches successfully.

19 June 1976

Viking 1 enters orbit around Mars.

20 July 1976

After a hoped-for landing on 4 July to celebrate the US Bicentennial is postponed, the Viking 1 orbiter releases its lander, which parachutes into the atmosphere and lands successfully in the Chryse Planitia region.

7 August 1976

Viking 2 arrives at Mars.

3 September 1976

Viking 2's lander touches down in Utopia Planitia.

Early Mars missions

The first Mariner flybys of Mars had suggested that that the planet was a barren, cratered ball of rock, but throughout the 1970s orbiters and landers revealed a far more complex, fascinating world.

Although Mariners 4, 6, and 7 had all made successful flybys of Mars in the 1960s, no spaceprobe had yet gone into orbit around the planet - but this would change with the Mariner 9 mission. Launched by an Atlas-Centaur rocket, this probe (with its failed twin, Mariner 8) was the first to be equipped with a retrorocket that would allow it to lose excess speed and drop into orbit around Mars. When it arrived after a six-month journey in November 1971, the Red Planet was enveloped in one of its periodic global dust storms, but as the atmosphere began to clear over the following weeks, an unexpected landscape emerged. The northern hemisphere was dominated by huge volcanoes towering above

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Mariner 9 was different from earlier Mariner probes in several respects - it had larger solar arrays to generate power and a retrorocket to slow it down as it reached Mors.

FIRST ORBITER

Mariner 9 was different from earlier Mariner probes in several respects - it had larger solar arrays to generate power and a retrorocket to slow it down as it reached Mors.

smooth lowland plains. These included a peak which was later named Olympus Mons - the largest volcano in the Solar System, some 500km (300 miles) across and rising to 27km (17 miles) above the average Martian surface. Perhaps even more impressive was the deep canyon system called the Valles Marineris (Mariner Valleys). These huge scars in the Martian surface dwarf Earth's own Grand Canyon. They stretch for more than 4,000km (2,500 miles) around the Martian equator and are more than 10km (6 miles) deep and 600km (375 miles) across in places. Unlike their Earthly equivalent, it was clear that these valleys were produced by faults in the Martian crust, not a result of water erosion - but elsewhere, Mariner 9 photographed winding valleys, "islands", and outflow channels that looked very much like the result of water flowing in the planet's ancient past.

How had the three previous probes so misled the scientists? By a fluke of their flightpaths, each had photographed only the rocky, cratered highlands dominating the southern hemisphere - Mars is a world of two halves, and until 1971 its interesting side had been missed.

Outposts on Mars

In 349 days of operation, Mariner 9 transformed the scientists' image of Mars - now it seemed that the

VIEWS FROM THE GROUND

The Viking landers found variations in the terrain around the planet. Viking Lander 1's location (below) had fewer rocks and more windblown sand dunes. Viking Lander 2's landing site (right) was strewn with many more rocks, resembling Earth's volcanic basalts.

VIKING'S VIEW

The Viking orbiters compiled a comprehensive photographic atlas of Mars, allowing the production of mosaics of entire hemispheres. This view is dominated by the Valles Marineris canyon system and huge volcanoes on the left-hand limb.

VIKING'S VIEW

The Viking orbiters compiled a comprehensive photographic atlas of Mars, allowing the production of mosaics of entire hemispheres. This view is dominated by the Valles Marineris canyon system and huge volcanoes on the left-hand limb.

VIEWS FROM THE GROUND

The Viking landers found variations in the terrain around the planet. Viking Lander 1's location (below) had fewer rocks and more windblown sand dunes. Viking Lander 2's landing site (right) was strewn with many more rocks, resembling Earth's volcanic basalts.

7 August 1976

Viking 2 arrives at Mars.

3 September 1976

Viking 2's lander touches down in Utopia Planitia.

TECHNOLOGY

THE VIKING ORBITERS AND LANDERS

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The Viking orbiters were based on the successful Mariner template but considerably enlarged. Cameras and other instruments were mounted on a movable platform attached to an octagonal body 2.4m (100in) across. The body held most of the probe's electronics, while an antenna beamed data from the orbiter and lander back to Earth. On the back of this octagon was a rocket motor that slowed the probe down as it arrived at Mars, while the lander itself, sealed inside a sterile outer shell, was attached to the front. The landers were thoroughly sterilized before launch to prevent contamination of the Martian surface or the biology experiments with organic material or bacteria from Earth. Once at Mars, Viking's operators used images from the orbiter to study potential landing sites before releasing the lander.

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high-gain antenna for direct contact with Earth propellont tank.

possibilities of water and even life at some point in Martian history were back on the agenda. The advanced Viking missions, in development since 1969 and now scheduled for launch in 1975, suddenly became a focus of intense interest.

Twin Viking spacecraft were planned, each with two elements - an orbiter and a lander (see panel, above). Viking 1 arrived in orbit around Mars in June 1976, while Viking 2 arrived in August. The orbiters continued to photograph the entire planet at relatively high resolutions until 1980, but attention soon switched to the landers, which parachuted to the ground in July and September. The pictures they sent back recorded the landscapes of the Chryse Planitia and Utopia Planitia regions respectively, and in each case, the general impression was of a landscape of orange-red sand littered with rocks beneath a salmon-pink sky. As well as cameras, the landers carried other equipment to study the surface. This included seismometers to detect any Martian earthquakes, a weather station that reported temperatures ranging from -120°C (-184°F) at night to -14°C (7°F) in the day, and a sampler arm to scoop up and study Martian soil.

Most attention, however, focused on the biology processor - a set of three experiments that looked for evidence of photosynthesis, bacteria, or just organic (carbon-based) matter in the soil samples. While there was no sign of photosynthesis at work, and the tests for organic matter also drew a blank, the soil did appear to react when "fed" with nutrients. Most scientists thought this was due to unusual chemistry rather than life, but the results were inconclusive. However, it would be a long time before another probe was able to continue Viking's work.

radioisotope thermal generator ™ power source high-gain antenna for direct contact with Earth meteorology sensors landing shock-

absorber propellont tank.

radioisotope thermal generator ™ power source meteorology sensors landing shock-

absorber

surface sampler boom biology processor

MARS VIKING LANDER

To slow its descent through the thin Martian atmosphere, each Viking lander used parachutes at first and then retrorockets. The lander's sterile shell also shielded it during atmospheric entry, before falling away during the final approach. Onboard antennae allowed for communications with Earth, either sending signals directly or relaying them through the orbiter.

surface sampler boom biology processor

MARS VIKING LANDER

To slow its descent through the thin Martian atmosphere, each Viking lander used parachutes at first and then retrorockets. The lander's sterile shell also shielded it during atmospheric entry, before falling away during the final approach. Onboard antennae allowed for communications with Earth, either sending signals directly or relaying them through the orbiter.

2 March 1972

Pioneer 10 is launched on its 20-month long journey to Jupiter.

5 April 1973

Pioneer 11 sets out on its journey to Saturn via Jupiter.

3 December 1973

Pioneer 10 makes the first flyby of Jupiter.

5 March 1979

Voyager 1 makes its flyby of Jupiter. Voyager 2 follows four months later.

1 September 1979

Pioneer 11 makes the first flyby of Saturn.

12 November 1980

Voyager 1 flies through the Saturn system and makes a close approach to Titan.

25 August 1981

Voyager 2 flies past Saturn and heads on to Uranus and Neptune.

24 January 1986

Voyager 2 makes the first flyby of Uranus.

25 August 1989

Voyager 2 makes the first flyby of Neptune.

Voyages among giants

The successful use of a gravity assist by Mariner 10 opened the way for ambitious missions to cross the vast distances between the giant outer planets of the Solar System.

The space in which the inner, Earthlike worlds of the

Solar System orbit is dwarfed by the orbits of the four outer planets. Even the closest of these giants,

Jupiter, orbits 5 astronomical units (AU) from the Sun (an astronomical unit is the average radioi-

distance between Earth and the Sun, gener roughly 150 million km or 93 million miles). /

The outermost giant, Neptune, is ^ l-fi^'

some 30 AU away from the Sun. L

The giants are huge balls of gas surrounded by large families of moons and moonlets - complex systems that were an obvious scientific target for space probes. But their great distance placed them beyond the range of easy targets for Space Race spectaculars - their exploration would be a long-term endeavour.

NASA launched two initial probes, Pioneers 10 and 11, towards Jupiter and Saturn in 1972 and 1973 respectively. These became the first objects to cross the asteroid belt between Mars and Jupiter, proving that it was largely empty space and the risk of a collision was small. Pioneer 10 reached its target in December 1973, and sent back pictures that improved on Earthbound telescopes, but still left many questions unanswered. Pioneer 11 swung past Jupiter in 1974 and used the giant planet's gravity high-gain antenna_

high-gain antenna_

particle experiments radioisotope thermal generator (RTG)

particle experiments radioisotope thermal generator (RTG)

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VOYAGER

The twin Voyager probes bear a distinct resemblonce to their Mariner ancestors. Stabilized by small thrusters, they orient themselves in space by tracking the Sun and the bright star Canopus.

PIONEER AND ITS PLAQUE

The Pioneer probes (right) to Jupiter and Saturn were to be the first human objects sent to go beyond the Solar System, and the mission scientists felt it was important to send a greeting to any alien civilisation that might one day find them. To this end, they devised a plaque (above) that shows a man and a woman, along with basic directions to reach Earth.

PIONEER AND ITS PLAQUE

The Pioneer probes (right) to Jupiter and Saturn were to be the first human objects sent to go beyond the Solar System, and the mission scientists felt it was important to send a greeting to any alien civilisation that might one day find them. To this end, they devised a plaque (above) that shows a man and a woman, along with basic directions to reach Earth.

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wide- and narrow-angle cameras

main system bus with infrared / propulsion system and ultraviolet instruments to swing it towards Saturn, which it reached in 1979. By this time, a more sophisticated pair of probes was already following in its wake.

Planning the Voyagers

During their initial studies of gravity assist in the 1960s, scientists had noticed that the giant planets would fall into a particularly neat arrangement in the late 1970s. A spacecraft launched during this alignment, which only occurs once every 176 years, would be able to make a "grand tour" of the outer Solar System, receiving from each giant planet in turn a gravity boost that would alter its course and accelerate it towards its next target. The opportunity was too good to miss, and NASA began to develop plans for a Thermoelectric Outer Planets Spacecraft (TOPS). However, as the mission grew more ambitious and complex, its budget spiralled, leading to its cancellation amid the cutbacks of 1972.

With just a few years until the launch window, NASA went back to the drawing board, designing a simpler probe based on Mariner technology. By the time the mission was ready in 1977, the twin spacecraft had developed so far from Mariner that they were given a new name - Voyagers.

Grand tourists

The Voyager mission design called for Voyager 1 to travel on a faster trajectory than its sibling, and so Voyager 2 was launched first, on 20 August 1977, and Voyager 1 took off 15 days later. Voyager 1

ERUPTIONS ON 10

As Voyager 1 turned for a last look back at Jupiter's innermost large moon, it captured this image of a huge cloud rising over the satellite's limb. It proved to be a plume of sulphurous chemicals erupting from a volcano on the surface.

ERUPTIONS ON 10

As Voyager 1 turned for a last look back at Jupiter's innermost large moon, it captured this image of a huge cloud rising over the satellite's limb. It proved to be a plume of sulphurous chemicals erupting from a volcano on the surface.

swung past Jupiter on 5 March 1979, followed in July by its twin. The probes sent back the first close-up images of the large moons lo, Europa, Ganymede, and Callisto. These revealed active sulphur volcanoes on lo, an icy crust with hints of a hidden ocean on Europa, and a thin ring of dust around Jupiter itself.

At Saturn, the paths of the Voyagers diverged. Voyager 1 swung close to the planet, photographing its famous rings and flying past the giant moon Titan, which proved to a smoggy orange atmosphere. Voyager 2 flew past further out, using gravity assist to sling it on towards an encounter with Uranus in 1986. Here it found an eerily placid green world and an array of moons, including the bizarre Miranda with a surface that seems to be a jumble of different terrain types.

The final leg of the probe's marathon journey carried it on to Neptune in 1989. This proved to be a far more active world, with dark storms and high winds raging in its blue atmosphere. Its large satellite, Triton, was even stranger - with geysers

TECHNOLOGY

SLINGSHOTS ACROSS THE SOLAR SYSTEM

of liquid nitrogen and a surface temperature of -238°C (-396°F). Leaving Neptune behind in its wake, Voyager 2 headed into the outer limits of the Solar System - like Voyager 1 and its Pioneer siblings, it is moving fast enough to leave the Solar System forever and wander among the stars.

1 V2 Jupiter arrival, July 1979

Gravity assist put strict constraints on the flightpaths of the Voyager probes. The main goals of the mission were to revisit Jupiter and Saturn and get a good look at Jupiter's four large satellites and Saturn's moon Titan. Uranus and Neptune were an optimistic afterthought, but a close Titan flyby would make it impossible for a probe to continue towards them, so mission designers came up with a contingency plan. Voyager 1 took a fast route to Saturn and Titan, while Voyager 2 followed a slower path that could be diverted to Titan if something went wrong. Fortunately nothing did, and the extension of Voyager 2's 2 VI Saturn arrival, mission was authorized shortly November 1980 after its Saturn encounter.

2 V2 Saturn arrival, August 1981

4 V2 Neptune arrival, August 1989

1 V2 Jupiter arrival, July 1979

2 V2 Saturn arrival, August 1981

4 V2 Neptune arrival, August 1989

1 VI Jupiter arrival, March 1979

is now more than IS billion km (9.3 billion miles) from Earth

1 VI Jupiter arrival, March 1979

is now more than IS billion km (9.3 billion miles) from Earth

STORMS ON NEPTUNE

So far out in the Solar System, Neptune was expected to be a placid, deep-frozen world. Instead, Voyager 2 found huge storm systems and some of the highest wind speeds in the Solar System - all powered by an unknown internal energy source.

Magellan Arrives Venus

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4 May 1989

NASA's Magellan probe is deployed from the Space Shuttle Atlantis. Because of scheduling difficulties that arose in the aftermath of the Challenger disaster, Magellan has to follow a long route to Venus, taking 15 months.

10 August 1990

Magellan arrives at Venus and enters a near-polar orbit, circling the planet every 3 hours 15 minutes.

15 May 1991

The probe completes its primary 243-day mission, mapping more than 80 per cent of the Venusian surface, and then enters an extended phase of operations.

14 September 1992

With three mapping cycles complete, Magellan begins a new phase of its mission, in which variations in its orbit are used to map Venusian gravity.

May 1993

Magellan lowers its orbit using the new technique of aerobraking.

11 October 1994

Magellan is deliberately plunged into the atmosphere of Venus at the end of its successful five-year mission.

MAGELLAN SPACECRAFT

The main 3.5m (11'/¿ft) dish on the Magellan orbiter (shown here under construction at the Martin Marietta factory in Denver) played dual roles. During half of each orbit around Venus, it received SAR signals transmitted by the cone-like horn antenna (on the left of the dish). For the other half of its orbit, the probe turned to face Earth and the dish beamed back the data it had gathered.

Mapping Venus

The task of charting what lay beneath the choking Venusian atmosphere required an ingenious application of Earth-based remote-sensing techniques to the problems of a more distant world.

While the Soviet Union had little luck further afield in the Solar System, their probes dominated the exploration of Venus in the 1970s and 1980s. The only US effort of the period was the Pioneer Venus mission of 1978 (see p.261). However, this did introduce a new technique to solve the \ problem of Venus's perpetual cloud cover. Radar had first been used to study Venus from Earth in 1961, but Pioneer brought a radar instrument to Venus itself for the first time.

The idea behind radar altimetry is simple - radio waves are fired from a spacecraft at the ground below, and a receiver on the spacecraft picks up their reflected echoes. Because the radio waves travel at a known speed (the speed of light), it is easy to calculate the distance of the ground from the time the signal takes to return. In Earthbound applications, the principle is used to calculate the distance of a spacecraft from tracking stations at known locations, but the method can be reversed - if the craft's orbit is known precisely, then the

VENUS UNVEILED

A spherical projection of Magellan data, with colours inspired by the Venera surface images, shows what Venus might look like with its clouds stripped away.

reflection time can reveal the height of the terrain below - [r; ; • t\ mountain peaks will return A echoes sooner than deep

The Pioneer studies _

w were followed by two

Soviet orbiters, Veneras 15 ^B and 16. A vague picture was emerging of a low, flat planet, with a few high plateaus and widespread volcanic peaks. These were given names, jj such as Alpha Regio and the Maxwell Montes, but astronomers were still eager to see more detail of the Venusian landscape, and that would require another, more sophisticated probe.

The Magellan mission

NASA's original successor to Pioneer Venus was VOIR (Venus Orbiting Imaging Radar). This would have been a huge spacecraft fitted with six different instruments. When costs and complexities began to spiral, the project was cancelled, to be replaced by a stripped-down Venus Radar Mapper (VRM) mission. In 1986, VRM was renamed Magellan, in honour of the famous Portuguese explorer.

Magellan carried an ingenious synthetic aperture radar (SAR), using the same principle as normal radar but producing maps with far higher resolution (see panel, opposite). New techniques allowed the science team on Earth to extract even more information from a ' the radar data - as well as basic topography, they could measure the

RARE IMPACT

Magellan showed that impact craters are rare on Venus. The thick atmosphere means that most incoming objects burn up before impact, so craters like Dickinson, 69km (43 miles) across, are amongst the smallest to form.

TECHNOLOGY

steepness and roughness of the terrain and even identify different rock types based on the strength with which they reflected Magellan's radar.

In order to map as much of Venus as possible, Magellan entered an orbit that took it over the planet's poles. Each orbit generated a narrow strip of radar data wrapping around the planet, and, as Venus slowly rotated beneath the probe, a picture of the entire surface built up. By the end of three Venusian rotations (each 243 Earth days long), the probe had mapped 98 per cent of the surface of

MAPPING VENUS

Magellan data was used to produce detailed global maps of Venus, such as these of the northern (left) and southern hemispheres. Highland areas are shown in cream and white, lowlands in blue and purple.

Venus at a resolution of around 100m (330ft). With Magellan far outlasting its expected life, the next three 243-day mapping cycles were used to build up a model of variations in the planet's surface gravity. During this phase of the mission, the probe's orbit was gradually lowered until it skimmed the upper atmosphere and lost a little of its speed each time - a rehearsal for a fuel-saving technique called aerobraking (see p.274). As a finale, Magellan made a dramatic death plunge towards the surface, its radio signals revealing the properties ^

of Venus's upper atmosphere.

SIF AND GULA MONS

Altimetry data and other information from Magellan were combined to produce 3-D visualizations and flyovers of the Venusian terrain. This area of the Eistlo Regio highlands is dominated by two huge volcanoes and a vast plain of solidified lava.

More often used on Earth satellites, SAR uses the speed at which a satellite travels along its orbit to produce radar images with higher resolutions. Instead of a single "ping", each SAR pulse is a longer "chirp" that varies its frequency over time. As the echoes of the chirp return to the satellite, it moves along a significant track through space. By identifying which parts of the chirp returned at what point on the track, a detailed picture of the ground below can be generated.

-gain antenna high-gain antenna (shown fully open)

RTG boom, 5m (16ft Sin) long bus Sunshade for protection in inner Solar System

thruster retropulsion module baffle to shield spacecraft electronics from RIG radiation scan platform

number of instruments 10 on orbiter spacecraft height 5.3m (17ft 5in)

DEVELOPMENT TESTS

Before any spaceprobe is built, numerous mock-ups are produced to test how it might operate in outer space. Here, a model of Galileo is shown in its launch configuration at the JPL facility.

one of two Radioisotope Thermoelectric Generators (RTGs) produces 250 watts of power for spacecraft

Radioisotope Thermoelectric Generator

READY TO GO

Galileo stands in Kennedy Space Center's Vertical Processing Facility (VPF), ready for mating with its Inertial Upper Stage booster before its 1989 launch.

TECHNOLOGY

NASA'S PROBE TO JUPITER

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